The present invention relates to a method of producing high purity aluminum-lithium mother alloys and more particularly to a method of producing aluminum-lithium mother alloys which are substantially free of contamination by calcium and alkali metals such as sodium, potassium, etc., other than lithium.
Generally, aluminum-lithium mother alloys have been heretofore produced by the method involving the following two basic steps.
(1) electrolytic production of metallic lithium; and
(2) melting and casting.
In step (1), metallic lithium is produced by electrolysis of a molten salt mixture consisting of lithium chloride and potassium chloride. In step (2), the metallic lithium produced in the step (1) is added, in an amount needed to provide the desired mother alloy composition, to aluminum and melted together to obtain cast ingots of the mother alloys.
As high purity aluminum-lithium mother alloys suitable for use in practical applications, it is requested that they contain lithium in an amount of 10 wt.% or more and that sodium and potassium be each not more than 5 ppm and calcium be not more than 10 ppm.
Currently, commercially available electrolytic lithium with a high purity of 99.9% includes approximately 200 ppm sodium, 100 ppm potassium and 200 ppm calcium and thus it is impossible to produce high purity aluminum-lithium mother alloys using such lithium. Further, in order to produce superhigh purity electrolytic lithium with sodium not exceeding 50 ppm, an additional purification process of lithium salts or metallic lithium is needed.
When the purification of lithium is carried out by means of molten metal treatment with gas, serious loss of lithium is unavoidably occurs. Further, current efficiencies in the electrolysis of lithium in the conventional methods are relatively low, for example, 70 to 90% at most.
Further, in the conventional methods of producing aluminum-lithium mother alloys, remelting of the electrolytic lithium and aluminum is indispensable in the foregoing step (2), and in this remelting process, lithium is liable to deteriorate due to its extremely high activity. In order to prevent such an unfavorable deterioration, the remelting must be carried out under a controlled atmosphere of inert gas. Further, lithium tends to cause an unfavorable segregation in the course of solidification because of its low melting point and small density. Therefore, it is very difficult to produce constantly mother alloys with stable desired compositions by the conventional methods.
As a method to overcome the disadvantages hereinbefore mentioned, Assignee has previously developed a production process as discussed in U.S. Pat. No. 4,521 284, issued June 4, 1985 which process is characterized in that solid aluminum cathodes are used in the electrolytic preparation of aluminum-lithium mother alloy. In this process, alloying proceeds from a cathode surface toward a cathode central part with the progress of electrolysis and, in this procedure, expansion simultaneously occurs in the cathodes. With the progress of the expansion, cracks occur in the alloyed portion and gradually become larger, thereby causing serious problems as set forth below.
(1) When removing the resulting alloy, an electrolytic bath enters the cracks. PA1 (2) Electrolyzing operation can not be stably performed due to variation in cathode current density. PA1 (3) The produced ally is liable to fall off from the cathode surfaces. PA1 (4) Large area is occupied by cathodes.